In recent years, many proposals have been made for orthogonal frequency division multiplexing (OFDM) which is a modulation technique for transmitting digital signals. With OFDM modulation, a large number of orthogonal subcarriers are provided in a transmission band and data are allocated to the amplitude and the phase of each subcarrier for digital modulation using PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation). With OFDM modulation, since a transmission band is divided by a large number of subcarriers, the bandwidth of each subcarrier is rather small to make the modulation rate low, although the overall transmission rate is comparable to that of any known modulation method. Additionally, with OFDM modulation, the symbol transmission rate is low because a large number of subcarriers are transmitted in parallel. Because of these characteristics, it is possible with OFDM modulation to reduce the relative multipath time length relative to the symbol time length to make the transmission less vulnerable to multipath interference. Still additionally, with OFDM modulation, since data are allocated to a plurality of subcarriers, the transmission/reception circuit can be formed by using an inverse fast Fourier transform (IFFT) operation circuit for modulation and a fast Fourier transform (FFT) operation circuit for demodulation.
In view of the above identified characteristics of OFDM modulation, efforts are being paid to apply it to ground wave digital broadcasting and communication that are apt to be strongly affected by multipath interference.
More specifically, standards for terrestrial digital broadcasting employing OFDM modulation have been proposed, including the DVB-T (Digital Video Broadcasting-Terrestrial) Standard and the ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) Standard.
With OFDM modulation, signals are transmitted on the basis of a unit of symbol referred to as OFDM symbol as shown in FIG. 1 of the accompanying drawings. For signal transmission, an OFDM symbol is made to comprise an effective symbol covering a signal period good for IFFT and a guard interval formed by copying a rear part of the effective symbol. The guard interval is arranged at the head of the OFDM symbol. According to the DVB-T Standard (2K mode), for instance, the effective. symbol contains 2,048 subcarriers arranged with regular intervals of 4.14 kHz. Data are modulated on 1,705 subcarriers out of the 2,048 subcarriers in an effective symbol. The guard interval has a time length equal to ¼ of that of the effective symbol.
Firstly, a known OFDM modulator will be described below.
Referring to FIG. 2 of the accompanying drawings, the known OFDM modulator 101 comprises a MUX adaptation/energy dispersion circuit 102, a Reed-Solomon encoder 103, a convolutional interleave circuit 104, a convolutional encoder 105, a bit/symbol interleave circuit 106, a mapping circuit 107, a frame adaptation circuit 108, an IFFT circuit 109, a guard interval adding circuit 110, a D/A converter 111, a frond end 112, an antenna 113 and a TPS generation circuit 114.
The OFDM modulator 101 receives as input an MPEG-2 transport stream formed by compressing and multiplexing video and audio signals by means of an upstream MPEG encoder. The transport stream is supplied to the MUX adaptation/energy dispersion circuit 102 of the OFDM modulator 101.
The MUX adaptation/energy dispersion circuit 102 bit-inverts the sync byte 47 h that is the leading byte of TS packet once in every eight TS packets to turn it to B8h. At this time, it initializes the shift register for generating a pseudo-random number series (PRBS) that is used for energy dispersion once in every eight TS packets by using a seed. According to the DVB-T Standard, the PRBS is (x15+x14+1) and the seed is 009 Ah. The MUX adaptation/energy dispersion circuit 102 operates for energy dispersion by performing an exclusive OR operation of the data excluding the sync byte (1 byte) of the TS packet and the PRBS. The data series that has been subjected to energy dispersion is supplied to the Reed-Solomon encoder 103.
The Reed-Solomon encoder 103 performs a Reed-Solomon coding operation on the input data series and adds a parity of 16 bytes for each TS packet. The data series to which a parity is added is supplied to the convolutional interleave circuit 104.
The convolutional interleave circuit 104 performs an interleaving operation on the input data series. For example, the convolutional interleave circuit 104 has 12 branches that are provided with respective delay elements having respective amounts of delay that are different from each other as shown in FIG. 3. It selects a same branch for both input and output, switching branches for every byte sequentially in a manner such as 0, 1, 2, 3, 4, . . . , 10, 11, 0, 1, 2, . . . . It outputs a byte for input a byte and performs a convolutional interleaving operation. The data series subjected to the convolutional interleaving operation is then fed to the convolutional encoder 105.
The convolutional encoder 105 performs convolutional coding by means of two encoders such as G1=171 (Octal) and G2=133 (Octal) and outputs encoded 2 bits for a 1-bit input. When it performs a puncturing operation, it does so on the output of encoded 2 bits. The data series subjected to convolutional coding is then fed to the bit/symbol interleave circuit 106.
The bit/symbol interleave circuit 106 interleaves the frequency in the OFDM symbol and the bits allocated to mapping points. The interleaved data series is then supplied to the mapping circuit 107.
The mapping circuit 107 divides the data series by a code length conforming to the employed modulation method (e. g., a code length of 6 bits for 64 QAM) and allocates the divided data series to predetermined mapping points. As a result of allocating the data series to the mapping points, two-dimensional information comprising I and Q components is output. The data series that is turned to two-dimensional information is supplied to the frame adaptation circuit 108.
The frame adaptation circuit 108 performs a so-called OFDM framing operation of inserting a predetermined pilot signal, a transmission line multiplexing control signal (IPS: Transmission Parameter Signaling) and a null signal fed from the TPS generation circuit 114 into the mapped two-dimensional information. The data series subjected to OFDM framing is then fed to the IFFT circuit 109.
The IFFT circuit 109 turns the 2,048 sets of data for I and Q to an OFDM symbol and performs an IFFT operation collectively on it. The data series subjected to an IFFT operation is then supplied to the guard interval adding circuit 110 on an effective symbol by-effective symbol basis.
The guard interval adding circuit 110 makes a copy of the signal waveform of the rear ¼ of the signal of each effective symbol output from the IFFT circuit 109 and adds the copy to the head of the effective symbol to make it a guard interval. The data series now added with a guard interval is then supplied to the D/A converter 111.
The D/A converter 111 converts the digital signal into an analog signal and supplies the latter to the front end 112.
The front end 112 up-converts the frequency of the analog signal obtained by the D/A conversion to the RF band and transmits it into air by way of the antenna 113.
Now, a known OFDM demodulator will be described below by referring to FIG. 4 of the accompanying drawings.
As shown in FIG. 4, the known OFDM demodulator 131 comprises an antenna 132, a tuner 133, an A/D converter 134, a digital orthogonal demodulation circuit 135, an FFT operation circuit 136, a narrow band fc error computation (FAFC) circuit 137, a broad band fc error computation circuit 138, a numerical-controlled oscillation (NCO) circuit 139, an equalizer 140, a demapping circuit 141, a TPS (Transmission Parameter Signaling) demodulation circuit 142, a bit/symbol deinterleave circuit 143, a Viterbi decoding circuit 144, a convolutional deinterleave circuit 145, a Reed-Solomon decoding circuit 146 and a MUX adaptation/energy inverse dispersion circuit 147.
The wave transmitted from the broadcasting station for digital television broadcasting is received by the antenna 132 of the OFDM decoder 131 and fed to the tuner 133 as RF signal.
The tuner 133 transforms the frequency of the RF signal received by the antenna 132 and outputs an IF signal. The output IF signal is then fed to the A/D converter 134.
The A/D converter 134 digitizes the IF signal. The digitized IF signal is then supplied to the digital orthogonal modulation circuit 135. According to the DVB-T Standard (2K mode), the A/D converter 134 quantizes the effective symbol and the guard interval of a so-called OFDM time region signal with a double clock typically for sampling respectively 4,096 samples and 1,024 samples.
The digital orthogonal demodulation circuit 135 performs orthogonal demodulation on the digitized IF signal, using the carrier signal of a predetermined frequency (carrier frequency) and outputs a base band OFDM signal. The base band OFDM signal output from the digital orthogonal demodulation circuit 135 is a so-called time region signal that is to be subjected to an FFT operation. Therefore, a base band signal that has been subjected to digital orthogonal demodulation and yet is to be subjected to an FFT operation is referred to as OFDM time region signal hereinafter. As a result of orthogonal demodulation, the OFDM time region signal becomes a complex signal containing a real axis component (I channel signal) and an imaginary axis component (Q channel signal).
The OFDM time region signal output from the digital orthogonal demodulation circuit 135 is then supplied to the FFT operation circuit 136 and the narrow band fc error computation circuit 137.
The FFT operation circuit 136 performs an FFT operation on the OFDM time region signal and extracts the orthogonal-modulated data on each subcarrier, which data is then output. The signal output from the FFT operation circuit 136 is a so-called frequency region signal that has been subjected to FFT. Therefore, a signal that has been subjected to an FFT operation is referred to as OFDM frequency region signal hereinafter.
The FFT operation circuit 136 extracts the signals in an effective symbol length (e. g., 2,048 samples) out of an OFDM symbol. In other words, it extracts signals from the part of an OFDM symbol obtained by excluding the guard interval. Then, it performs an FFT operation on the OFDM time region signal of the extracted 2,048 samples. More specifically, the operation starting position will be found between the boundary of the OFDM symbol (position at A in FIG. 1) and the end position of the guard interval (position at B in FIG. 1). This range of operation is referred to as FFT window.
Like the OFDM time region signal, the OFDM frequency region signal output from the FFT operation circuit 136 is a complex signal containing a real axis component (I channel signal) and an imaginary axis component (Q channel signal). The OFDM frequency region signal is then supplied to the broad band fc error computation circuit 138 and the equalizer 140.
The narrow band fc error computation circuit 137 computes the carrier frequency error contained in the OFDM time region signal. More specifically, the narrow band fc error computation circuit 137 computes the narrow band carrier frequency error with an accuracy of ±½ of the subcarrier frequency interval (4.14 kHz) or less. The carrier frequency error is the error of the central frequency position of the OFDM time region signal that can be produced typically by displacement of the reference frequency output from the local oscillator of the tuner 133. The error rate of the output data increases when this error becomes large. The narrow band carrier frequency error determined by the narrow band fc error computation circuit 137 is then fed to the NCO 139.
The broad band fc error computation circuit 138 computes the carrier frequency error contained in the OFDM time region signal. More specifically, the broad band fc error computation circuit 138 computes the broad band carrier frequency error with an accuracy of the subcarrier frequency interval (4.14 kHz) or less. The broad band fc error computation circuit 138 refers to a continual pilot signal (CP signal) and computationally determines the extent, or the amount of shift, by which the CP signal is shifted from the proper insertion position of the CP signal. The broad band carrier. frequency error determined by the broad band fc error computation circuit 138 is supplied to the NCO 139.
The NCO 139 adds the narrow band carrier frequency error of the accuracy of ±½ of the subcarrier frequency interval as determined by the narrow band fc error computation circuit 137 and the broad band carrier frequency error of the accuracy of the subcarrier frequency interval as determined by the broad band fc error computation circuit 138 and outputs a carrier frequency error correction signal whose frequency increases/decreases as a function of the carrier frequency error obtained as a result of the addition. The carrier frequency error correction signal is a complex signal and supplied to the digital orthogonal demodulation circuit 135. The digital orthogonal demodulation circuit 135 performs digital orthogonal demodulation, correcting the carrier frequency fc according to the carrier frequency error correction signal.
The equalizer 140 equalizes the phase and the amplitude of the OFDM frequency region signal, using a scattered pilot signal (SP signal). The OFDM frequency region signal whose phase and amplitude are equalized is then supplied to the demapping circuit 141 and the TPS demodulation circuit 142.
The TPS demodulation circuit 142 separates the TPS signal assigned to a predetermined frequency component and demodulates the information containing the coding ratio, the modulation method, the guard interval length and so on from the signal.
The demapping circuit 141 performs a demapping operation on the OFDM frequency region signal whose phase and amplitude have been equalized by the equalizer 140 according to the modulation method to decode the data. The demapped data is then fed to the bit/symbol deinterleave circuit 143.
The bit/symbol deinterleave circuit 143 performs an operation exactly opposite to that of bit-interleaving and symbol-interleaving conducted by the modulator. The data that is subjected to bit-deinterleaving and symbol-deinterleaving is then supplied to the Viterbi decoding circuit 144.
The Viterbi decoding circuit 144 performs a maximum likelihood decoding operation, using the Viterbi algorithm. The data subjected to maximum likelihood decoding is then supplied to the convolutional deinterleave circuit 145.
The convolutional deinterleave circuit 145 operates oppositely relative to the convolutional interleave circuit of the modulator. The data subjected to convolutional deinterleaving is then fed to the Reed-Solomon decoding circuit 146.
The Reed-Solomon decoding circuit 146 decodes the Reed-Solomon code according to the parity of the 16 bytes added by the modulator and corrects errors, if any. The data subjected to Reed-Solomon decoding is then fed to the MUX adaptation/energy inverse dispersion circuit 147.
If the sync byte of the TS packet that is the leading byte is 47 h, the MUX adaptation/energy inverse dispersion circuit 147 does nothing on it. However, if the sync byte is B8h, it inverts the bits and modifies the byte to 47 h. At this time, the MUX adaptation/energy inverse dispersion circuit 147 initializes the shift register for generating a pseudo-random number series (PRBS) that is used for energy dispersion at every TS packet whose sync byte is B8h by means of a predetermined seed. According to the DVB-T Standard, the PRBS is (x15+x14+1) and the seed is 009 Ah. The MUX adaptation/energy inverse dispersion circuit 147 operates for energy inverse dispersion by performing an exclusive OR operation of the data excluding the sync byte (1 byte) of the TS packet and the PRBS. The data series that has been subjected to energy inverse dispersion is supplied typically to a downstream MPEG-2 decoder as transport stream.
Meanwhile, wireless cameras are being used for live new reports, live sports coverages and live coverages of various events of television broadcasting. Wireless television cameras provide advantages over cabled television cameras including non-need of cabling and de-cabling operations and freedom of selection of camera angles and shooting positions to improve the mobility of cameras on site because signals of the images and the sounds taken up by the cameras are transmitted wirelessly by means of ground waves to the base station that may be an outside broadcast van.
Additionally, the video signals and audio signals obtained by the shooting operation of the wireless camera are digitized and transmitted to the base station by using a digital modulation method.
However, a plurality of broadcasting organizations may report independently from a site. If the signals are transmitted wirelessly in such a situation, they may be received not only by the staff of the reporting broadcasting organization but also by the third party that may be the staff of other broadcasting organizations. The signals may include those of the picked up raw images and sounds as well as those of auxiliary information.